The present invention relates to an adjustable valve gear and, more particularly, to an adjustable valve gear for at least two lift-type intake valves per cylinder in an internal combustion engine having at least one camshaft with cams, each of which actuates a respective rocker arm via a first curve joint situated in a central region thereof, each of which rocker arm is supported via a second curve joint in a region of a longitudinal end thereof on an adjusting shaft, with a support position adjustable as a function of operating parameters including speed and load of the internal combustion engine by rotation of the adjusting shaft in an adjustment path, and which is articulated in a region of another longitudinal end on a respective rocker cam pivotable about a pivot supported on a housing of the internal combustion engine and rests on the respective lift-type intake valve via a third curve joint comprising a respective base-circle track and an adjoining cam track on the rocker cam.
An adjustable valve gear is shown in DE 38 31 642 A1. In this known valve gear, the two lift-type intake valves of a cylinder are actuated via a common rocker cam, with the result that the two valve lift curves of the lift-type intake valves are always the same as one another.
DE 33 32 789 C2 and DE 29 49 529 A1 disclose other types of adjustable valve gears having two lift-type intake valves per cylinder, in which the valve lift curve of one lift-type valve is unchangeable, while the phase relation of the valve lift curve of the other lift-type valve can be changed to particular end positions and intermediate positions without a change in the lift height and the lift duration being possible.
An object on which the present invention is based is to better match in an adjustable valve gear the valve lift curves to the operating conditions of the internal combustion engine with a low outlay, without sacrificing the possibility of a change in the lift duration, the lift height and the phase relation of the valve lift curves.
The foregoing object has been achieved according to the present invention by providing that each lift-type intake valve is operatively associated with a separate rocker cam and at least one of the three curve joints associated with one of the lift-type intake valve differs from the similar curve joint associated with another lift-type intake valves of the same cylinder such that the lift curve of the one lift-type intake valve differs from the lift curve of the other lift-type intake valve in the same position of the adjusting shaft over at least part of the adjustment path of the adjusting shaft.
In the case of the valve gear configured in accordance with the present invention, it is possible, while retaining just one adjusting shaft and, if required, just one camshaft, (i.e., with little outlay) for the two lift-type intake valves to achieve different valve lift curves for the same operating conditions of the internal combustion engine while retaining the possibilities of adjustment as regards the phase relation, lift duration and lift height. It is thus possible to match the opening of the lift-type intake valves optimally to any operating condition of the internal combustion engine.
The different valve lift curves can be achieved in accordance with the present invention by different configurations of one or more curved tracks forming the curve joints or by identical configuration but arrangement of the curved tracks in a manner rotated relative to one another. In the former, there is greater freedom as regards the differing configuration of the valve lift curves while, in the latter, the outlay is further reduced by the production of identical surfaces and parts.
The present invention also makes possible, without any further outlay, operation of the internal combustion engine with just one opening lift-type intake valve. This is advantageous particularly at low speed and load, or complete shut-down of a cylinder.
The present invention further results in a particularly small space requirement for the valve gear overall while retaining all possibilities of changing of the intake-valve lift curves since a separate rocker-lever bearing shaft is not required and all valves can be actuated via a single camshaft.